The recovery of aromatics from gasoline fractions and refinery streams is an important process step in petrochemistry and coke oven and refinery technology. Especially benzene and simple derivatives of benzene are important raw materials in the production of dyestuffs, plastics, solvents and varnishes. As these compounds in aromatics-containing fractions frequently occur in mixtures with non-aromatic compounds, process steps for their isolation are of great importance. Examples of fractions containing aromatics are reformate gasoline and pyrolysis gasoline but also distillation fractions from mineral oils or coke-oven light oil.
It is not possible to simply separate aromatics from gasoline fractions containing aromatics by way of distillation since the gasoline or its fractions consist of a large variety of substances of very similar boiling ranges. The separation is therefore to be carried out by processes which take advantage of other physical effects. The technical implementation can be achieved by several processes based on different physical separation processes. To be mentioned here in the first place are the azeotropic distillation, the liquid/liquid extraction and the extractive distillation.
In the azeotropic distillation, a solvent is added to the mixture to be separated, this solvent forming a mixture of constant boiling point with the aliphatic or the aromatic component. This azeotrope is separated from the starting mixture by distillation and after distillation fractionated into azeotrope former and aromatic fraction. In the liquid/liquid extraction, the mixture to be separated is provided with a solvent generating a two-phase mixture and involving a higher solubility of one component and thus extracting it from the solvent mixture. The aromatic component may be separated from the solvent, for example, by way of distillation after the extraction.
The extractive distillation takes advantage of the phenomenon that there is a change in the fugacities in a mixture of appropriate components. The fugacity is here to be understood as the corrected partial vapour pressure in the mixture. Reason for the change in fugacity is the fact that there are different repellent interactions between the individual types of molecules. A mixture component that has stronger repulsive forces than the other components will change into the vapour phase more easily than a component of lower repulsive forces.
In an extractive distillation a solvent is added which is known to be capable of selectively increasing the fugacity of one or several components. In the case of hydrocarbon mixtures containing aromatics, the aliphatic components of the mixture frequently have stronger repulsive forces vis-à-vis the solvent so that their fugacity is hence considerably increased. In contrast to this, the fugacity of the aromatic component changes comparatively less. For this reason, a distillation with the solvent will effect that the aliphatic components are preferably obtained in the raffinate, the low-boiling head product of a distillation, whereas the aromatic components are obtained in the extract, the higher-boiling bottom product of a distillation. This makes it necessary to use a solvent achieving the desired effect by changing the fugacities of the individual components in the desired way.
An extractive distillation frequently has advantages over an azeotropic distillation or a liquid/liquid extraction. The mass transfer in an extractive distillation frequently is considerably higher than in an azeotropic distillation as, in the case of the former, the applied temperatures are distinctly higher. An extractive distillation requires considerably less equipment than a liquid/liquid extraction, as only two distillation columns are usually required instead of one extraction column with downstream distillation unit. As considerably less solvent is required for the extractive distillation as compared to a liquid/liquid extraction, the costs of installation and operation are notably lower.
The central problem to be solved when performing an extractive distillation is the selection of a suitable solvent. From the large number of possible solvents the one is to be determined that allows the intended separation with a minimum amount of circulated solvent. Decisive criteria for this are the capacity and the selectivity of a solvent. The capacity indicates how the aromatic component in liquid state is distributed among the individual phases according to Nernst's distribution law. The higher the capacity, the better the solubility of the aromatic component in the solvent and the lower the solvent demand. The lower the capacity, the higher is the probability that a two-phase mixture is formed with the aromatic components and the solvent in liquid state. Hence the capacity mainly determines how much solvent is required.
The selectivity indicates the improvement of the extraction of the desired transition component in comparison to the other components contained in the raffinate. The higher the selectivity of an extracting agent, the stronger is the repulsion of the aliphatic component and thus the corresponding change in fugacity. The selectivity essentially determines the separating efficiency and thus the number of theoretical trays required for the extractive distillation. The lower the selectivity, the more equipment is required.
Various solvents suited for an extractive distillation of aromatics are known. Frequently used solvents are diethylene glycol, dimethylsulfoxide, sulfolane, n-methylpyrrolidone, dimethylacetamide and n-formyl morpholine. A hydrocarbon stream is used as a feed mixture, which contains aromatic and aliphatic components and is distilled in a pre-distillation unit to give a hydrocarbon with a relatively narrow boiling point range. Depending on the separating efficiency of the extractive distillation, the feed stream is a C6-stream, a (C6-C7)-stream or a (C6-C8)-stream.
The actual equipment for the extractive distillation usually consists of two distillation columns. The first column serves to perform the actual extractive distillation. At the head of the column a raffinate stream is obtained which predominantly consists of non-aromatic hydrocarbons and, depending on the configuration, a certain amount of solvent. As the repulsive effect of the solvent is stronger for the non-aromatic hydrocarbons, these compounds change to the vapour phase more easily. At the lower part of the column a mixture is obtained which predominantly consists of aromatic compounds and the extracting solvent. This mixture is then passed into a stripping column where the aromatics-containing mixture is separated from the solvent by way of distillation. The solvent is recycled back to the first column.
The distillative separation of the extract gives a hydrocarbon mixture rich in aromatics as a fraction at the head of the stripping column and a solvent fraction as bottom product which is lean in aromatics. Both fractions may be passed to a downstream purification. Once purified, the aromatics may, for instance, be further processed by distillation to obtain the individual aromatic compounds by alkylation degree and boiling point. Thus the benzene derivatives benzene, toluene and xylene are obtained. To separate the xylene isomers, further process steps may follow. As a purification step for the aromatics fraction, a scrubbing process with water may be advisable.
DE 1568940 C3 describes a process for the extractive distillation of aromatics using n-formyl morpholine as a solvent. The process can be used for the isolation of aromatics from a starting fraction containing aromatics as well as for the removal of aromatics from hydrocarbon streams. This process is run in a facility including a column for extractive distillation, a solvent stripper, a stripping column and a solvent regeneration column. Depending on purity and requirements, the aromatics contained can be obtained either directly or be submitted to further treatment. Owing to the relatively low solvent capacity this process requires a high amount of solvent and involves high constructional cost.
EP 679708 A1 describes an extraction process which requires only one extraction column owing to a specific equipment arrangement. The extraction is carried out in a column from which a head product rich in aliphatic compounds is obtained and a side product which is rich in aromatics from the middle column section. The solvent is recycled from the bottom via heat-exchanging devices to the upper column section. Both hydrocarbon streams are freed from excessive solvent and water in cyclone separators and downstream phase separators. As solvents, preferably polyalkylene glycols are used but also sulfolanes or pyrrolidones. To improve the separating efficiency, 0.1 to 20 mass percent of water are added to the solvent mixture. A disadvantage of this process is that the separating efficiency can only be improved if a certain amount of water is added. This makes it necessary to install additional devices for drying the products obtained.
EP 1280869 B1 describes a process for the extractive distillation of a hydrocarbon mixture containing aromatics using a solvent mixture of sulfolane and 3-methyl sulfolane. The solvent mixture can be used in any ratio desired and thus adapted to the aromatics content and the composition of the aromatics portion to an optimum degree. This process is run in a facility consisting of a column for extractive distillation and a column for distillation of the aromatic fraction. By such arrangement the process can be run with a relatively low demand for equipment. The disadvantage of this process is a large amount of circulated solvent and a relatively large column for extractive distillation, as a large portion of extracting solvent combination as compared to the hydrocarbon must be used.